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Since the axes of the cylinders are coplanar, the connecting rods cannot all be directly attached to the crankshaft unless mechanically complex forked connecting rods are used, none of which have been successful. Instead, the pistons are connected to the crankshaft with a master-and-articulating-rod assembly. One piston, the uppermost one in the animation, has a master rod with a direct attachment to the crankshaft. The remaining pistons pin their connecting rods' attachments to rings around the edge of the master rod. Extra "rows" of radial cylinders can be added in order to increase the capacity of the engine without adding to its diameter.

Four-stroke radials have an odd number of cylinders per row, so that a consistent every-other-piston firing order can be maintained, providing smooth operation. For example, on a 5-cylinder engine the firing order is 1,3,5,2,4 and back to cylinder 1. Moreover, this always leaves a one-piston gap between the piston on its combustion stroke and the piston on compression. The active stroke directly helps compressing the next cylinder to fire, so making the motion more uniform. If an even number of cylinders was used, the equally timed firing cycle would not be feasible. [1] The protoype radial Zoche aero-diesels (below) have an even number of cylinders, either four or eight; but this is not problematic, because they are two-stroke engines, with twice the number of power strokes as a four-stroke engine.

The radial engine normally uses fewer cams compared to other types. As with most four-strokes, the crankshaft takes two revolutions to complete the four strokes of each piston (intake, compression, combustion, exhaust). The camshaft ring is geared to spin slower and in the opposite direction to the crankshaft. The cam lobes are placed in two rows for the intake and exhaust. For the example, 4 cams serve all 5 cylinders, whereas 10 would be required for a typical inline engine with the same number of cylinders and valves.

In 1903–1904 Jacob Ellehammer used his experience constructing motorcycles to build the world's first air-cooled radial engine, a three-cylinder engine which he used as the basis for a more powerful five-cylinder model in 1907. This was installed in his triplane and made a number of short free-flight hops.[3]

Another early radial engine was the three-cylinder Anzani, originally built as a W3 "fan" configuration, one of which powered Louis Blériot's Blériot XI across the English Channel. By 1914 Anzani had developed radial engines ranging from 3 cylinders (spaced 120° apart) to a massive 20-cylinder engine of 200 hp (150 kW), with its cylinders arranged in four groups of five.[2]

Radial engines are regarded as being air-cooled almost by definition — so that it is interesting that one of the most successful of the early radial engines was the Salmson 9Z series of nine-cylinder water-cooled radial engines that were produced in large numbers during the First World War. Georges Canton and Pierre Unné patented the original engine design in 1909, offering it to the Salmson company — and the engine was often known as the Canton-Unné.

From 1909 to 1919 the radial engine was overshadowed by its close relative, the rotary engine — which differed from the so-called "stationary" radial in that the crankcase and cylinders revolved with the propeller. Mechanically it was identical in concept to the later radial except that the propeller was bolted to the engine, and the crankshaft to the airframe. The problem of the cooling of the cylinders, a major factor with the early "stationary" radials was solved by the engine generating its own cooling airflow.

Little development of the radial engine was undertaken in Germany during World War I, where most aircraft used water-cooled inline 6-cylinder engines. The German Oberursel firm made licensed copies of the Gnome and Le Rhône rotary powerplants while Siemens-Halske built a number of their own designs including the Siemens-Halske Sh.III eleven-cylinder rotary engine, which was unusual for the period in being geared down, so that the engine could spin at a higher speed than the propeller, and in the opposite direction.

By the end of the war the rotary engine had reached the limits of the design, particularly in regard to the amount of fuel and air that could be drawn into the cylinders during the intake stroke due to the rotary motion, while advances in both metallurgy and cylinder cooling finally allowed stationary radial engines to supersede rotary engines. In the early 1920s Le Rhône converted a number of their rotary engines into stationary radial engines although most of the early radial engines were new.

In the United States, the National Advisory Committee for Aeronautics (NACA) noted in 1920 that air-cooled radials could offer an increase in the power-to-weight ratio and reliability, and by 1921 the U.S. Navy had announced it would only order aircraft fitted with air-cooled radials while other naval air arms followed suit. Charles Lawrance's J-1 engine was developed in 1922 with Navy funding, and using aluminium cylinders with steel liners ran for an unprecedented 300 hours, at a time when 50 hours endurance was normal. At the urging of the Army and Navy the Wright Aeronautical Corporation bought Lawrance's company, and subsequent engines were built under the Wright name. The radial engines gave confidence to Navy pilots performing long-range overwater flights.[4]

Wright's 225 hp (168 kW) J-5 Whirlwind radial engine of 1925 was widely acknowledged as "the first truly reliable aircraft engine".[5] Wright employed Giuseppe Mario Bellanca to design an aircraft to showcase it, and the result was the Wright-Bellanca 1, or WB-1, which was first flown in the latter part of that year. The J-5 was used on many advanced aircraft of the day, including Charles Lindbergh's Spirit of St. Louis with which he made the first solo trans-Atlantic flight.

In 1925, the American rival firm to Wright's radial engine production efforts, Pratt & Whitney, was founded. The P & W firm's initial offering, the Pratt & Whitney R-1340 Wasp, test run later that year, began the evolution of the many models of Pratt & Whitney radial engines that were to appear during the second quarter of the 20th century, among them the 14-cylinder, twin-row Pratt & Whitney R-1830 Twin Wasp, the most-produced aviation engine of any single design, with a total production quantity of nearly 175,000 engines.

In the United Kingdom the Bristol Aeroplane Company was concentrating on developing radials such as the Jupiter, Mercury and sleeve valveHercules radials. France, Germany, Russia and Japan largely built licenced or locally improved versions of the Armstrong Siddeley, Bristol, Wright, or Pratt & Whitney radials.

Liquid-cooled engines often weigh more and their cooling systems are both more complex and are generally more vulnerable to battle damage. Minor shrapnel damage easily results in a loss of coolant and consequent engine seizure, while an air-cooled radial would be unaffected.[7] Additionally, radials have shorter and stiffer crankshafts, a single bank radial needing only two crankshaft bearings as opposed to the seven required for a six-cylinder inline engine of similar stiffness.[8] The shorter crankshaft also produces less vibration and hence higher reliability through reduced wear.

While a single bank radial permits all cylinders to be cooled equally, the same is not true for multi-row engines where the rear cylinders are affected by the heat coming off the front row, and air flow being masked.[9] Additionally, having the cylinders in the airflow increases drag considerably, adding turbulence that destroys the laminar airflow over the fuselage and adjacent wings. The answer to both these problems was the addition of specially designed cowlings with baffles to force the air over the cylinders. The first effective drag reducing cowling that didn't impair engine cooling was the British Townend ring or "drag ring" which formed a narrow band around the engine covering the cylinder heads, not only reducing drag, but adding a small amount of thrust. National Advisory Committee for Aeronautics then studied the problem, developing the NACA cowling which further reduced drag, increased thrust and improved cooling. Nearly all aircraft radial engine installations since have used NACA type cowlings.
The thrust generated by both the Townend ring and the NACA cowling was due to the Meredith Effect (discovered by British researchers) which was also put to use in the radiators of several aircraft that used liquid-cooled engines such as the Spitfire and Mustang.[10]

Because each cylinder on a radial engine has its own head, it is impractical to use a multivalve valvetrain on a radial engine. Therefore, almost all radial engines use a two valve pushrod-type valvetrain and as a result produce less power for a given displacement than multi-valve inline or vee engines.
The radial engine typically has a larger frontal area, and is less amenable to streamlining and drag reduction than an inline engine. Pilot visibility is often poorer due to the width of the engine, and the designer is limited in engine placement as ensuring adequate cooling air, either in a buried engine installation or in a pusher configuration is more difficult.

While inline liquid-cooled engines continued to be common in new designs until late in World War II, radial engines dominated afterwards until overtaken by jet engines, with the late-war Hawker Sea Fury and Grumman Bearcat, two of the fastest production piston-engined aircraft ever built, used radial engines. Until the development of the jet engine, large aircraft commonly used radial engines. Factors influencing the choice of radial over inline were reliability and simplicity in maintenance. The large frontal area of these aircraft meant the radial engine's own frontal profile was a less significant factor when it came to drag.

Originally radial engines had one row of cylinders, but as engine sizes increased it became necessary to add extra rows. The first known radial-configuration engine to ever use a twin-row design was the 160 hp Gnôme "Double Lambda" rotary engine of 1912, designed as a 14-cylinder twin-row version of the firm's 80 hp Lambda single-row seven-cylinder rotary, with only the German Oberursel U.III clone of the Double Lambda reproducing the Gnome Double Lambda's twin-row design before the end of World War I. Most stationary radial engines did not exceed two rows, but the largest displacement mass produced aircraft radial engine, the R-4360, with cylinders in corncob configuration, was a 28-cylinder 4-row radial engine used in a number of large American aircraft in the post-World War II period. The Soviet Union built a limited number of 'Zvezda' engines with 56 cylinders but aircraft engines of this size, power and complexity were made obsolete by large turboprop engines which easily exceeded them in power without the weight or complexity. Large radials continued to be built for other uses though, as 112-cylinder Diesel boat engines with 16 rows of 7 cylinders each displacing 383 liters (23,931 in3) and producing 10,000 hp (7,500 kW) were used on fast attack craft, such as Osa class missile boats.

A number of companies continue to build radials today. Vedeneyev produces the M-14P radial of 360 hp (270 kW) to 450 hp (340 kW)) as used on Yakovlev and Sukhoi aerobatic aircraft. The M-14P is also popular among builders of experimental aircraft, such as the Culp's Special, and Culp's Sopwith Pup[1], Pitts S12 "Monster" and the Murphy "Moose". 110 hp (82 kW) 7-cylinder and 150 hp (110 kW) 9-cylinder engines are available from Australia's Rotec Engineering. HCI Aviation offers the R180 5-cylinder (75 hp (56 kW)) and R220 7-cylinder (110 hp (82 kW)), available "ready to fly" and as a build-it-yourself kit. Verner Motor of the Czech Republic now builds several radial engines ranging in power from 71 hp (53 kW) to 172 hp (128 kW). Miniature radial engines for model airplanes are available from Seidel in Germany, OS and Saito Seisakusho of Japan, and Technopower in the USA.

While most radial engines have been produced for gasoline, there have been Diesel radial engines. Two major advantages favour Diesel engines — lower fuel consumption and reduced fire risk.

Packard designed and built a 9-cylinder 980 cubic inch displacement Diesel radial aircraft engine, the 225 horsepower (168 kW) DR-980, in 1928. On 28 May 1931, a DR-980 powered Bellanca CH-300 piloted by Walter Edwin Lees and Frederick Brossy set a record for staying aloft for 84 hours and 32 minutes without being refueled.[11] This record stood for 55 years until broken by the Rutan Voyager.[12]

The experimental Bristol Phoenix of 1928–1932 was successfully flight tested in a Westland Wapiti and set altitude records in 1934 that lasted until World War II.

In 1932 the French company Clerget developed the 14D, a 14-cylinder two-stroke Diesel radial engine. After a series of improvements, in 1938 the 14F2 model produced 520 hp (390 kW) at 1910 rpm cruise power, with a power-to-weight ratio near that of contemporary gasoline engines and a specific fuel consumption of roughly 80% that for an equivalent gasoline engine. During WWII the research continued, but no mass-production occurred because of the Nazi occupation. By 1943 the engine had grown to produce over 1,000 hp (750 kW) with a turbocharger. After the war, the Clerget company was integrated in the SNECMA company and had plans for a 32-cylinder Diesel engine of 4,000 hp (3,000 kW), but in 1947 the company abandoned piston engine development in favour of the emerging turbine engines.

The Nordberg Manufacturing Company of the United States developed and produced a series of large two-stroke radial Diesel engines from the late 1940s for electrical production, primarily at aluminium smelters and for pumping water. They differed from most radials in that they had an even number of cylinders in a single bank (or row), thanks to an unusual double master connecting rod that allowed the engine to be timed so the cylinders fired in consecutive order. Variants were built that could be run on either Diesel oil or gasoline or mixtures of both. A number of powerhouse installations utilising large numbers of these engines were made in the U.S.[13]

In the years leading up to World War II, as the need for armoured vehicles was realised, designers were faced with the problem of how to power the vehicles, and turned to using aircraft engines, among them radial types. The radial aircraft engines provided greater power-to-weight ratios and were more reliable than conventional inline vehicle engines available at the time. This reliance had a downside though: if the engines were mounted vertically as in the M3 Lee and M4 Sherman, their comparatively large diameter gave the tank a higher silhouette than designs using inline engines.

A number of multi-cylinder 4-stroke model engines have been commercially available in a radial configuration, beginning with the Japanese O.S. Max firm's FR5-300 five-cylinder, 3.0 cu.in. (50 cm3) displacement "Sirius" radial in 1986. The American "Technopower" firm had made smaller-displacement five- and seven-cylinder model radial engines as early as 1976, but the OS firm's engine was the first mass-produced radial engine design in aeromodeling history. The rival Saito Seisakusho firm in Japan has since produced a similarly sized five-cylinder radial four-stroke model engine of their own as a direct rival to the OS design, with Saito also creating a trio of three-cylinder radial engines ranging from 0.90 cu.in. (15 cm3) to 4.50 cu.in. (75 cm3) in displacement. The German Seidel firm has made both seven- and nine-cylinder "large" (starting at 70 cm3 displacement) radio control model radial engines, mostly for glow plug ignition, with an experimental fourteen-cylinder twin-row radial being tried out.